contrib: Use asmap for ASN lookup in makeseeds

Add an argument `-a` to provide a asmap file to do the IP to ASN
lookups.

This speeds up the script greatly, and makes the output deterministic.
Also removes the dependency on `dns.lookup`.

I've annotated the output with ASxxxx comments to provide a way to
verify the functionality.

For now I've added instructions in README.md to download and use the
`demo.map` from the asmap repository. When we have some other mechanism
for distributing asmap files we could switch to that.

This continues #24824. I've removed all the fallbacks and extra
complexity, as everyone will be using the same instructions anyway.

Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>
Co-authored-by: James O'Beirne <james.obeirne@pm.me>
Co-authored-by: russeree <reese.russell@ymail.com>

1 files changed, 90 insertions, 0 deletions

diff --git a/contrib/seeds/asmap.py b/contrib/seeds/asmap.py
new file mode 100644
index 0000000000..e7e05a1d10
--- /dev/null
+++ b/contrib/seeds/asmap.py
@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+# Copyright (c) 2013-2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+import ipaddress
+
+# Convert a byte array to a bit array
+def DecodeBytes(byts):
+    return [(byt >> i) & 1 for byt in byts for i in range(8)]
+
+def DecodeBits(stream, bitpos, minval, bit_sizes):
+    val = minval
+    for pos in range(len(bit_sizes)):
+        bit_size = bit_sizes[pos]
+        if pos + 1 < len(bit_sizes):
+            bit = stream[bitpos]
+            bitpos += 1
+        else:
+            bit = 0
+        if bit:
+            val += (1 << bit_size)
+        else:
+            for b in range(bit_size):
+                bit = stream[bitpos]
+                bitpos += 1
+                val += bit << (bit_size - 1 - b)
+            return (val, bitpos)
+    assert(False)
+
+def DecodeType(stream, bitpos):
+    return DecodeBits(stream, bitpos, 0, [0, 0, 1])
+
+def DecodeASN(stream, bitpos):
+    return DecodeBits(stream, bitpos, 1, [15, 16, 17, 18, 19, 20, 21, 22, 23, 24])
+
+def DecodeMatch(stream, bitpos):
+    return DecodeBits(stream, bitpos, 2, [1, 2, 3, 4, 5, 6, 7, 8])
+
+def DecodeJump(stream, bitpos):
+    return DecodeBits(stream, bitpos, 17, [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30])
+
+def Interpret(asmap, num, bits):
+    pos = 0
+    default = None
+    while True:
+        assert(len(asmap) >= pos + 1)
+        (opcode, pos) = DecodeType(asmap, pos)
+        if opcode == 0:
+            (asn, pos) = DecodeASN(asmap, pos)
+            return asn
+        elif opcode == 1:
+            (jump, pos) = DecodeJump(asmap, pos)
+            if (num >> (bits - 1)) & 1:
+                pos += jump
+            bits -= 1
+        elif opcode == 2:
+            (match, pos) = DecodeMatch(asmap, pos)
+            matchlen = match.bit_length() - 1
+            for bit in range(matchlen):
+                if ((num >> (bits - 1)) & 1) != ((match >> (matchlen - 1 - bit)) & 1):
+                    return default
+                bits -= 1
+        elif opcode == 3:
+            (default, pos) = DecodeASN(asmap, pos)
+        else:
+            assert(False)
+
+
+
+def decode_ip(ip: str) -> int:
+    addr = ipaddress.ip_address(ip)
+    if isinstance(addr, ipaddress.IPv4Address):
+        return int.from_bytes(addr.packed, 'big') + 0xffff00000000
+    elif isinstance(addr, ipaddress.IPv6Address):
+        return int.from_bytes(addr.packed, 'big')
+
+class ASMap:
+    def __init__(self, filename):
+        '''
+        Instantiate an ASMap from a file.
+        '''
+        with open(filename, "rb") as f:
+            self.asmap = DecodeBytes(f.read())
+
+    def lookup_asn(self, ip):
+        '''
+        Look up the ASN for an IP, returns an ASN id as integer or None if not
+        known.
+        '''
+        return Interpret(self.asmap, decode_ip(ip), 128)